Vinylamine unit-containing polymers are widely used in flocculants, papermaking chemicals, fiber treatment agents, and the like. As the method for producing vinylamine unit-containing polymers, various kinds of production methods are known, a method to hydrolyze the carboxylic acid amide group (—NHC(═O)R (where, R is a hydrogen atom, a hydrocarbon group, or the like)) in an N-vinylcarboxylic acid amide unit-containing polymer is useful from the industrial point of view, and a method to hydrolyze the formamide group (—NHC(═O)H) in an N-vinylformamide unit-containing polymer is especially useful from the viewpoint that it is relatively easy to conduct the hydrolysis and it is easy to convert to a vinylamine unit-containing polymer (Patent Literatures 1 and 2).
The following methods are known as the production method of N-vinylformamide that is a raw material of an N-vinylformamide unit-containing polymer.
(1) A method to obtain N-vinylformamide by thermal decomposition of N-methoxyethylformamide (Patent Literature 3).
(2) A method to obtain N-vinylformamide by thermal decomposition of N-cyanoethylformamide (Patent Literature 4).
In both methods of (1) and (2), the starting material is formamide, and thus formamide is mixed in N-vinylformamide obtained by thermal decomposition of N-methoxyethylformamide or N-cyanoethylformamide. Hence, the following problems are caused in a case in which an N-vinylformamide unit-containing polymer is produced by using crude N-vinylformamide containing formamide and a vinylamine unit-containing polymer is produced through hydrolysis of the formamide group in the N-vinylformamide unit-containing polymer.
Formamide is mixed in the vinylamine unit-containing polymer to be obtained as well. It is not preferable that formamide is mixed from the viewpoint of quality in the case of using the vinylamine unit-containing polymer as a papermaking chemical, a fiber treatment agent, and the like.
Formamide has a greater chain transfer constant than water, and thus the molecular weight of an N-vinylformamide unit-containing polymer obtained by polymerizing N-vinylformamide in the presence of formamide is lower than that of an N-vinylformamide unit-containing polymer obtained by polymerizing N-vinylformamide in the absence of formamide.
Accordingly, in general, crude N-vinylformamide containing formamide is purified to obtain purified N-vinylformamide, an N-vinylformamide unit-containing polymer is then produced by using the purified N-vinylformamide, and a vinylamine unit-containing polymer is produced through hydrolysis of the formamide group in the N-vinylformamide unit-containing polymer. Examples of the purification method of crude N-vinylformamide may include a method by distillation and a method by extraction.
However, there are the following problems in the method by distillation.
The boiling point of formamide is close to the boiling point of N-vinylformamide, and thus multistage precision distillation (fractional distillation) accompanied by reflux is required in order to separate N-vinylformamide from formamide by distillation (Patent Literatures 5 and 6). For the multistage precision distillation, complicated and large-scale equipment (fractionator or the like) is required, and thus the purification of crude N-vinylformamide is not conveniently conducted by the method by distillation.
N-vinylformamide is unstable as compared to other vinylamides, especially it is thermally unstable, and thus it is required to be carefully handled. The yield of N-vinylformamide decreases by decomposition and the like in some cases in the case of attempting to obtain purified N-vinylformamide having a high purity by multistage precision distillation accompanied by reflux.
There is the following problem in the method by extraction.
The equipment for extraction is large and a great amount of solvent is required, and thus the method by extraction is not practical.